Britannica,' and he believed he should be enabled to impart an additional value to his writings by throwing upon them the light of Rennie's strong practical judgment. He proposed to take a lodging in the immediate neighbourhood of Rennie's house, then in the Great Surrey Road, and to board with him during the day; but Rennie would not listen to this proposal, and insisted on being the Professor's entertainer during the period of his visit.

One of the points which he particularly desired to discuss with Mr. Rennie was the theory of the equilibrium of arches a subject at that time very imperfectly understood, but which the young engineer had studied with his usual energy and success. He had clearly proved that the proper proportion and depth of the key-stone to that of the extrados (or exterior curve) should be in proportion to the size and form of the arch and the materials of which it was composed; and he had also established the ratio in which the arch-stones should increase from the key-stone to the piers or abutments. Up to this time there had been no rules laid down for the guidance of the engineer or architect, who worked very much in the dark as to principles; and it was often a matter entirely of chance whether a bridge stood or fell when the centres were removed. According to the views of Hutton and Attwood, the weight upon the haunches and abutments, to put the arch in a state of equilibrium so that it should stand, was unlimited; whereas Mr. Rennie established the limit to which the countervailing force or weight on the extrados should be confined. Hence he adopted the practice of introducing a flat inverted arch between the extrados of each two adjoining arches, (at the same time increasing the width of the abutment,)-the radii of the voussoirs or arch-stones being continued completely through them. And in order to diminish the masonry, the lower or foundation course was inclined also,-thus combining the work more completely together, and

enabling it better to resist the lateral thrust. Dr. Robison had much discussion with Mr. Rennie on these and many other points, and the information he obtained was shortly after worked up into numerous original contributions of great value; amongst which may be mentioned his articles in the Encyclopedia' on the Arch, Carpentry, Roof, Waterworks, Resistance of Fluids, and Running of Rivers-on all of which subjects Mr. Rennie had much original information to impart. It may readily be imagined that the evenings devoted by Dr. Robison to conversation and discussion on such topics at Rennie's house were of interest and advantage to both; and when the Doctor returned to his Edinburgh labours, he carried with him the cordial affection and respect of the engineer, who continued to keep up a correspondence with him until the close of his life.

In the early part of his career Mr. Rennie was called upon to furnish designs of many bridges, principally in Scotland, which, however, were not carried out, in most cases because the requisite funds could not be raised to build them. Thus, in 1798, he designed one of eight cast iron arches to span the river Don at Aberdeen. Four years later he was called upon to furnish further designs, when he supplied three several plans, two of granite bridges; but the structures were of too costly a character for the people of Aberdeen then to carry out. The first important bridge which Mr. Rennie was authorised to execute was that across the Tweed at Kelso, and it afforded a very favourable specimen of his skill as an architect. It was designed in 1799 and opened in 1803. It consists of five semi-elliptical arches of 72 feet span, each rising 28 feet, and four piers each 12 feet thick,

1 Dr. Robison was the first contributor to the Encyclopedia' who was really a man of science, and whose articles were above the rank of mere compilations. He sought information from all quarters-searched the works

of foreign writers, and consulted men of practical eminence, such as Rennie, to whom he could obtain access,-and extraordinary value was thus imparted to his articles.

with a level roadway 23 feet 6 inches wide between the parapets, and 29 feet above the ordinary surface of the river. The foundations were securely laid upon the solid rock in the bed of the Tweed, by means of coffer-dams, and below the deepest part of the river. The piers and abutments were ornamented with three-quarter columnar pilasters of the Roman Doric order, surmounted by a plain block cornice and balustrade of the same character. The whole of the masonry was plain rustic coursed work, and in style and execution it was long regarded as one of the most handsome and effective structures of its kind. It may almost be said to have formed the commencement of a new era of bridge-building in this country. The semi-elliptical arches, the columnar pilasters on the piers, the balustrade, and the level roadway, are the same as in Waterloo Bridge, except as regards size and character; so that Kelso Bridge may be regarded as the model of the greater work. We believe it was one of the first bridges in this country constructed with a level

roadway. Some of the old-fashioned bridges were excessively steep, and to get over them was like climbing the roof of a house. There was a heavy pull on one side and a corresponding descent on the other. The old bridge across the Esk at Musselburgh, forming part of the high road between Edinburgh and London, was of this precipitous character. It was superseded by a handsome and substantial bridge, with an almost level roadway, after a design by Rennie. When the engineer was taking the work off the hands of the contractor, one of the magistrates of the town, who was present, asked a countryman who was passing at the time with his cart how he liked the new brig? "Brig!" said the man, "it's nae brig ava! ye neither ken whan ye're on't, nor whan ye're aff't!"

Mr. Rennie's boldness in design grew with experience, and when consulted as to a bridge near Paxton, over the Whitadder (a rapid stream in Berwickshire), he

proposed, in lieu of the old structure, which had been carried away by a flood, a new one of a single arch, of 150 feet span; but unhappily the road trustees could not find the requisite means for carrying it into effect. Another abortive but grand design was proposed by him in 1801. He had been requested by the Secretary of State for Ireland to examine the road through North Wales to Holyhead, with the object of improving the communication with Ireland, which was then in a wretched state. The connection of the opposite shores of the Menai Strait by means of a bridge was considered an indispensable part of any improvement of that route; and Mr. Rennie proposed to accomplish this object by a single great arch of cast-iron 450 feet in span, the height of its soffit or crown to be 150 feet above high water at spring tides. A similar bridge, of 350 feet span, having its crown 100 feet above the same level, was also proposed by him for the crossing of Conway Ferry. These bridges were to be manufactured after a plan invented by Mr. Rennie in 1791, and communicated by him to Dr. Hutton in 1794; and he was strongly satisfied of its superiority to all others that had been proposed. The designs were alike

The great arch of 450 feet was to be supported on two stone piers, each 75 feet thick, the springing to be 100 feet above high water. There were to be arches of stone on the Caernarvon side to the distance of about 156 yards, and on the Anglesea side to the distance of about 284 yards; making the total length of the bridge, exclusive of the wing walls, about 640 yards. The estimated cost of the whole work and approaches was 268,500. The point at which the bridge was recommended to be thrown across was, either opposite Inys-yMoch island, on which one of the man piers would rest, or at the Swilly rocks, about 800 yards to the eastward; but, on the whole, he pre- | ferred the latter site. He also sent

in a subsequent design, showing an iron arch on each side of the main one of 350 feet span, in lieu of masonry, with other modifications, by which the dimensions of the main piers were reduced, and the estimate somewhat lessened. Other plans were prepared and submitted, embodying somewhat similar views, the prominent idea in all of them being the spanning of the strait by a great castiron arch, the crown of which was to be 150 feet above the sea at highwater. The plans and evidence on the subject are to be found set forth in the Reports from Committees of the House of Commons on Holyhead Roads (1810-22), ordered to be printed 25th July, 1822.